1. Field of the Invention
The present invention relates to a method for using a film formation apparatus for a semiconductor process, which is configured to form a film containing Ge (germanium) on a target object, such as a semiconductor wafer. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target object, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target object.
2. Description of the Related Art
Conventionally, for example, poly-silicon is used as a material for gate electrodes of transistors. Gate electrodes of poly-silicon can be easily depleted, when supplied with a bias voltage. As the thickness of gate insulating films decreases, this becomes prominent, which is one of the causes of device properties being deteriorated. In order to solve this problem, it has been studied to employ silicon germanium, which has a higher dopant activation rate, in place of silicon. It has also been studied to employ silicon germanium in other devices, such as diodes. For example, Jpn. Pat. Appln. KOKAI Publication No. 2003-77845 discloses a method for forming a silicon germanium film on the surface of a semiconductor wafer. This method employs a vertical heat processing apparatus, which supplies mono-silane (SiH4) gas and mono-germane (GeH4) gas to form a silicon germanium film by CVD (Chemical Vapor Deposition).
There is a case where one vertical heat processing apparatus is used to form a silicon germanium film on a first lot of wafers and then form anther thin film, such as a silicon film, on a second lot of wafers. In general, germanium contained in a process gas used in the preceding heat process can be a contaminant for a film, such as a silicon film, formed by the subsequent heat process. If germanium is mixed into the silicon film, device properties may be deteriorated.
Where the subsequent heat process is a silicon film formation process, a so-called pre-coating process is performed to cover the inner surface of the reaction container and so forth with a pre-coating film before the silicon film formation process. The pre-coating film prevents germanium from being scattered into the process atmosphere from by-product films, which contain silicon germanium as the main component (it means 50% or more), deposited inside the reaction container.
However, as described later, the present inventors have found that conventional methods for using a film formation apparatus for a semiconductor process of this kind have room for improvement in terms of problems concerning germanium contamination.